Game of Thrones season 1 started with King Robert Baratheon in his late middle-age and the death of Jon Arryn.
This prequel universe could start before Robert's rebellion and it will show us information from every house that we don't know!
House Stark: For the Starks we could learn about Ned's father and more information about Lyana and Benjen Stark.
House Baratheon: As for the Baratheons we could learn more about King Roberts' and King Stannis' childhood (Renly is a lot younger than them; he wouldn't even be born in the prequel).
House Targeryen: For the Targeryens we could learn about the Mad King, the youth of Rhaegar and if he had any connection with Lyana Stark and the childhood of Daenerys and Viserys!
House Greyjoy: We could see about Balon Greyjoy and his mini obsession about his crown and the independence of the iron islands
House Arryn: The original show began with the death of Jon Arryn the head of house Arryn and the hand of the King Robert. As for the Arryns we could learn his connection with Robert Baratheon and Eddard Stark, like the both of them said that Jon Arryn was like a father for them.
House Lannister: We could see about Casterly Rock, Tywin's Lannister youth and the childhood of Tyrion Lannister, Cercei Lannister and her beloved brother Jaime Lannister.
House Tyrell:We could learn more about Olenna Tyrell and her son Mace Tyrell
House Tully:About the Tullys we could learn about Hoster and Minisa Tully,parents of Catelyn's Stark/Tully and Lysa Arryn/Tully and the childhood of these two!
House Martell:About them we could learn about Doran's Martell,Oberyn's Martell and Ellia's Martell childhood
When Aerys II Targeryan was old enought,he started becoming a mad man,he thought he could become a dragon and he started burning people,lots of houses were afraid of him,but his son Rhaegar Targeryen kidnaps Lyana Stark sister of Eddard,Brandon and Benjen Stark and betrothed of Robert Baratheon,then Rickard and Brandon Stark visits king Aerys II and tells him what happened,after that king Aerys II became angry and burned Rickard alive for treason and imprisoned brandon Stark ,after that 4 houses(Arryn,Stark,Baratheon and Tully)started a rebellion against the mad king Aerys II Targeryen,after lots of fights Rhaegar and Robert are finally facing off each other with Robert killing Rhaegar with his war hammer,before that Lord Stannis Baratheon,was in a dillemma because he couldn't choose between his duty and his family,so he chose to help his brother Robert in the rebellion,the mad king was very scared of the rebellion and he wanted to create a huge fire to burn everyone even himself in order to become a dragon,then a member of the king's guard ser Jaime Lannister,was afraid of the mad king and chose to stab him in the back and kill him,the mad king's final words were:Burn them all!After Aerys'II death robert took the iron throne and the dynasty of the Targeryens ended there,after that King Robert gave a mission to Lord Stannis to kill the 2 left Targeryens,Daenerys and Viserys and conquer the house of the Targeryens,the Dragonstone,when lord Stannis arrived there it was too late,both of the Targeryen kids left,after that Robbert was very angry to stannis and he gave Storm's end to his younger brother Renly Baratheon.
The Greyjoy's Rebellion:
The rebellion took place nine years before the beginning of the series and was an attempt by House Greyjoy of the Iron Islands to break away and secure independence from the Iron Throne and revive the Old Way. Balon Greyjoy declared himself King of the Iron Islands and led the rebellion, supported by his vassals. The rebellion began eight years after King Robert I Baratheon seized the Iron Throne in the civil war known as Robert's Rebellion.During the rebellion, House Greyjoy secured several early victories, including the burning of the Lannister fleet at anchor in Lannisport in a raid led by Euron and Victarion Greyjoy.Their first major defeat came in an attempted attack on the coastal castle of Seagard. Balon's first son and heir, Rodrik, was killed in the failed assault.However, the superiority of numbers and resources saw the Iron Throne crush the rebellion. The Greyjoy fleet was destroyed by Lord Stannis Baratheon. King Robert and his warden of the North Lord Eddard Stark besieged the Greyjoy stronghold of Pyke. The Iron Islands were outnumbered ten to one. During the final assault on Pyke, a battle-crazed warrior priest of the Lord of Light named Thoros of Myr led the way through a breach in the wall with his flaming sword, Jorah Mormont of Bear Island not far behind him. Eddard Stark and Jaime Lannister were also key fighters for the Iron Throne during the battle.Balon's second son, Maron Greyjoy, was killed in the fighting at Pyke.Maron's death was due to a collapsing tower during the battle.Balon bends the knee, signalling the end of the rebellion.The rebellion was crushed and Balon was forced to surrender. He was accepted back into the king's peace, keeping his lordship and titles, but only on the condition that his last surviving son Theon be made a ward of House Stark as hostage for his good behavior. Theon was just 8 years old at the time. Robert's victory cemented his hold on the throne, after having overthrown the Targaryens a few years before. King Robert knighted Jorah Mormont for his valor in the battle. Jory Cassel also fought in the Greyjoy Rebellion.
Jon Arryn's death:
The prequel series should end when the original started,with jon Arryn's death by the lannisters.
His wife and Peter "Littlefinger"Baelish actually killed Jon Arryn